My U.S. Pat. No. 6,707,363 describes a magnetic structure based upon a conical geometry designed for NMR (sometimes referred to as MRI) head imaging in interventional procedures. The goals include an open structure providing full access to the head region while being imaged to a surgeon, minimize the dimensions and weight of the magnet materials, make an efficient use of the magnetic material with a maximum strength of the NMR imaging field, as well as allow access to the head of the patient from the vertical as well as the horizontal direction.
My improvement patent application, published as US 2004/0135580 A1, teaches several ways to improve the performance involving the use of magnetic straps, and a way to reduce the volume and weight of the permanent magnetic material by recessing a central pole piece to extend inwardly the outer permanent magnet. However, to achieve proper magnetic conditions at the material interfaces so as not to reduce the uniformity of the active magnetic field in the region of interest, it was thought necessary to introduce a non-magnetic region between the inwardly-extending magnetic section and a part of the pole piece and the remainder of the outer permanent magnet.
A further improvement patent application filed Oct. 27, 2003 by me, under application Ser. No. 10/694,419, features a structure wherein the interface between the pole piece and permanent magnetic conical section is configured to form an equipotential surface.
All three of these previous patent applications focused on head imaging, because it did not appear possible at the time those inventions were made with the structures there described to obtain a sufficiently large cavity having a uniform magnetic field in the region of interest to accommodate a larger body part, such as the chest of a patient.